One type of solid state capacitors, known as Metal-Insulator-Metal (MIM) capacitors, are basically formed from a dielectric disposed between two electrical conductors or electrodes, which can be layered as part of a semiconductor microchip. The area of each conductor, and the thickness and dielectric constant (k) of the dielectric, primarily establish the capacitance of the resulting capacitor. The process of forming the dielectric controls both the thickness and the dielectric constant for the dielectric. Generally once a dielectric is formed, its dielectric constant and the capacitance of the formed capacitor are fixed. Consequently, inefficient techniques for trimming capacitance after fabrication of the capacitor are employed. For example, multiple interconnected capacitors can be incorporated into a solid state structure and selectively cut out of the active circuit to trim the overall value of capacitance. This process, at best, provides a stepped value of capacitance trimming.
Known dielectrics for MIM capacitors are spun-on glasses, tetraethoxysilane (TEOS)/ozone films deposited at atmospheric or sub-atmospheric pressures, plasma deposited films either from silane (SiH4)/oxygen or TEOS reactions, or more recently, SiO2 films deposited by high density plasma chemical vapor deposition. The fixed dielectric constant of these deposited materials can range anywhere from 3.9 to approximately 5.0, largely depending on their density and how much moisture they absorb. Typical ranges for dielectric constants are: between 3.9 and 4.1 for plasma deposited silane oxides; between 4.1 and 4.3 for TEOS generated oxides; and 4.5 or higher for TEOS/ozone films.
It is known that introduction of fluorine in the above processes can produce dielectrics with a lower k value. This approach results in a fluorinated oxide, sometimes called fluorosilicate glass or, more accurately, a silicon oxyfluoride. Although there is a distinct tradeoff between the amount of fluorine added and the stability of the dielectric, the process can produce stable dielectrics with a k value around 3.5.
Most dielectrics with a k value less than 3.0 (low k dielectrics) fall into one of the following basic categories: fluorinated polyimides; non-polyimide C—H polymers; fluoro-polymers and siloxane polymers. Fabrication processing temperatures place additional limitations on low k dielectric selections from these categories at higher processing temperatures.
With respect to organosilicons that might serve as dielectrics, despite intensive research on the plasma deposition of amorphous silicon from monosilane (SiH4), there have been only a few reports exploring the formation of Si—Si bonded polymers from monosubstituted organosilanes. Haller reported an example of selective dehydrogenative polymerization, but no photochemical studies were described. See Haller, Journal of the Electrochemical Society A, Vol. 129, 1987, p. 180, and Inagaki and Hirao, Journal of Polymer Science A, Vol. 24, 1986, p. 595. Studies on the plasma chemistry of methylsilane (MeSiH3) have involved higher radio-frequency powers and temperatures which promote formation of amorphous silicon carbide (SiC) rather than reactive polymeric product. See Delpancke, Powers, Vandertop and Somorjai, Thin Solid Films, Vol. 202, 1991, p. 289. Low power plasma polymerization of tetramethylsilane and related precursors has been proposed to result in the formation of Si—C—Si linkages. See Wrobel and Wertheimer, Plasma Deposition, Treatment and Etching of Polymers, Academic Press, New York, Chapter 3. Such materials lack sufficient absorption in light above approximately 225 nm wavelength, but have been studied as far ultraviolet (193 nm wavelength) resists by Horn and associates. See Horn, Pang and Rothschild, Journal of Vacuum Science Technology B, Vol. 8, 1991, p. 1493. Polymer chemistry teaches the use of the basic silanes are insignificant as a monomer for polymerization type of polymer. Furthermore, polysiloxanes are differentiated from the basic silanes, and contrasted as being very important in terms of monomers for polymerization. See Stevens, Malcom P., Polymer Chemistry, An Introduction, Addison-Wesley Publishing Co., 1975: p. 334.
Work has been reported on the synthesis of soluble poly-alkylsilyne network polymers ([SiR]n) which exhibit intense ultraviolet absorption (associated with extended Si—Si bonding) and may be photo-oxidatively patterned to give stable siloxane networks. See Bianconi and Weidman, Journal of the American Chemical Society, Vol. 110, 1988, p. 2341. Dry development is accomplished by selective anisotropic removal of unexposed material by chlorine or hydrobromic acid reactive ion etching. See Hornak, Weidman and Kwock, Journal of Applied Physics, Vol. 67,1990, p. 2235, and Horn, Pang and Rothschild, Journal of Vacuum Science Technology B, Vol. 8, 1991, p. 1493. The exposed, oxidized material may be removed by either wet or dry fluorine based chemistry. Kunz and associates have shown that this makes polysilynes particularly effective as 193 nm wavelength photoresists. See Kunz, Bianconi, Horn, Paladugu, Shaver, Smith, and Freed, Proceedings of the Society of Photo-optical and Instrumentation Engineers, Vol. 218, 1991, p. 1466. The high absorbability and the wavelength limits photo-oxidation to the surface, eliminating reflection, and the pattern is transferred through the remainder of the film during the reactive ion etch (RIE) development. Studies of organosilicon hydride network materials containing reactive R—Si—H moieties have found that such high silicon compositions as [MeSiH0.5]n exhibit superior photosensitivity and function as single layer photodefinable glass etch masks. See Weidman and Joshi, New Photodefinable Glass Etch Masks for Entirely Dry Photolithography: Plasma Deposited Organosilicon Hydride Polymers, Applied Physics Letters, Vol. 62, No. 4, 1993, p. 372. However, cost and availability of the exotic organosilicon feedstocks have significantly inhibited the transfer of such photosensitive organosilicon hydride network materials into microcircuit fabrication. Further, films deposited from single component organosilicon feedstocks possess limited latitude in alteration of deposited film characteristics, such as the radiation frequency of photosensitivity and selectivity during etch processes.
An object of the present invention is a capacitor with a plasma deposited dielectric that has a low dielectric constant and good thermal stability at higher processing temperatures. Another object of the present invention is a capacitor with a plasma deposited dielectric that has a dielectric constant, and therefore, a value of capacitance, that can be changed after deposition of the dielectric or fabrication of the capacitor. Such capacitors are of particular advantage in the formation of electrical filters, among other applications, that require on-chip formation of precision value capacitors.